If you look in the forums, both here and other sites, you will see that a lot of admin's are having issues with this right now. I spent hours on the phones with RIM and they have been diging through our log files. There are some comon error mssgs that we are finding when someone tries to send a invite or a calendar event from the handheld. I think they need to look at this as a major issue and the tech's and dev people have said that there are quite a few common issues that can cause this....but more and more people are seeing this and the "known fixes" are not working.